The National Pension System (NPS) was introduced by the Central Government for its employees (excluding the Armed Forces) effective from January 1, 2004.

Why Option C is correct: Following the Central Government's implementation, various State Governments adopted the NPS architecture for their own employees. The scheme becomes applicable to State Government employees based on the specific date of notification issued by their respective State Governments.

Analysis of other options:

• Option A is incorrect: NPS is open to all Indian citizens, including both Resident Indians and Non-Resident Indians (NRIs).
• Option B is incorrect: The eligibility age for joining NPS is 18 to 70 years (it was 18 to 60 years at the time of the scheme's launch).
• Option D is incorrect: The NPS specifically excludes the Armed Forces. Furthermore, the implementation date for Central Government employees was January 1, 2004, not April 1, 2004.